A welding gun having a clamp mechanism for clamping workpiece to be welded during spot welding comprises an air servo cylinder formed by integrally assembling an air cylinder with servo valves, a controller for controlling the air cylinder, and the like. This welding gun is mounted on the tip of an arm of a robot for welding, and performs welding operation while being moved to various welding positions. Therefore, the requirements for the welding gun are small size, light weight, and compactness.
Also, it is desirable for the welding gun to be capable of clamping workpiece while being moved in narrow spaces, so that it is necessary to prevent air piping, an electric power supply line, and electric signal lines from becoming obstacles to operations of the welding gun.
In the air servo cylinder, when the servo valves are attached to the air cylinder, each of the discharge flow paths for air discharged from the air cylinder through the servo valve is generally provided with silencers for reducing exhaust noise. Conventionally, however, general-purpose silencers have been used to be installed so as to protrude from the discharge flow paths to the outside. This has increased the possibility of becoming obstacles to operations of the welding gun even if the silencers are small protrusions, since the silencers are attached to the welding gun so as to protrude toward the outside.
Such a problem is not confined to welding guns for spot welding, but generally occurs in various automatic operation apparatus that are attached to the tip of a robot, for working.
In the air servo cylinder, the pressure of each pressure chamber in the air cylinder is detected by a pressure sensor, and based on the detection signals, a control signal is outputted from the controller to each of the servo valves. Usually, the pressure in each of the pressure chambers is detected at a position of the servo valve adjacent to the air cylinder, and introduced into the controller through a signal line passing through the outside of the apparatus. This has caused a problem in that the air servo cylinder is subjected not only to sputtering occurring during welding but also to electromagnetic noises. It has therefore been necessary to provide a cover to the signal lines for avoiding influences of sputtering, and to take countermeasures against a malfunction caused by electromagnetic noises.